Reduction-fusion process for the production of rare earth intermetallic compounds

ABSTRACT

A reduction-fusion process for producing novel rare earth intermetallic compounds, for example, cobalt rare earth intermetallic compounds, especially compounds useful in preparing permanent magnets. A particular mixture of rare earth metal oxide and calcium hydride is heated to affect reduction of the rare earth metal oxide. The resulting rare earth metal-containing mixture is fused with cobalt or other ferromagnetic metal to form the rare earth intermetallic compound and allowed to solidify. The solid is pulverized and then treated to recover the rare earth intermetallic compound.

United States Patent [72] Inventor Robert E. Cech Scotia, N.Y. [211App]. No. 852,100 [22] Filed Aug. 21, 1969 [45] Patented Dec. 7, 1971[73] Assignee General Electric Company [54] REDUCTION-FUSION PROCESS FORTHE PRODUCTION OF RARE EARTH INTERMETALLIC COMPOUNDS 5 Claims, NoDrawings [52] US Cl 148/101, 75/84. 148/105 [5 1] Int. Cl H011 1/04,C22b 59/00 [50] FieldolSearch 148/101, 102,103, 105; 75/84, 152, 170,31.57

[56] References Cited UNITED STATES PATENTS 2,038,402 4/1936 Alexander75/84 2,584,411 2/1952 Alexander..... 75/84 X 3,104,970 9/1963 Downinget a1 75/152 X 3,424,578 1/1969 Strnat et a1. 75/170 UX 3,463,678 8/1969Becker 148/105 3,524,800 8/1970 Morrice et al, 75/152 X 2,043,363 6/1936Alexander 75/84 FOREIGN PATENTS 1,488,054 7/1967 France 75/152 6,608,33512/1967 Netherlands 148/3157 Primary Examiner-L. Dewayne RutledgeAssislan! Examiner-G. K. White Attorneys-Charles T. Watts, Paul A.Frank, Jane M.

Binkowski, Frank L. Neuhauser, Oscar B. Waddel and Joseph B. FormanABSTRACT: A reduction-fusion process for producing novel rare earthintermetallic compounds, for example, cobalt rare earth intermetalliccompounds, especially compounds useful in preparing permanent magnets. Aparticular mixture of rare earth metal oxide and calcium hydride isheated to affect reduction of the rare earth metal oxide. The resultingrare earth metal-containing mixture is fused with cobalt or otherferromagnetic metal to form the rare earth intermetallic ep npound andallowed to solidify. The solid is pulverized and then treated to recoverthe rare earth intermetallic compound.

REDUCTION-FUSION PROCESS FOR THE PRODUCTION OF RARE EARTH INTERMETALLICCOMPOUNDS RARE EARTH INTERMETALLIC COMPOUNDS PRODUCED BY AREDUCTION-FUSION PROCESS The present invention relates to rare earthintermetallic compounds, permanent magnets and a reduction-fusionprocess for preparing the compounds.

Permanent magnets, i.e. hard" magnetic materials, are of technologicalimportance because they can maintain a high, constant magnetic flux inthe absence of an exciting magnetic field or electrical current to bringabout such a field. A number of cobalt rare earth intermetalliccompounds, as for example C0,Sm, can be made into permanent magnets.However, these intermetallic compounds are not widely used in formingpermanent magnets because the methods of preparing these compounds arelengthy, time-consuming and costly. For example, a conventional processfor preparing a cobaltsamarium intermetallic compound useful as apermanent magnet comprises reduction of the samarium oxide by a numberof techniques such as heating the oxide with lanthanum metal chips in ahigh-temperature vacuum retort. On heating in vacuum the samarium oxideis reduced and, being more volatile than lanthanum, is vaporized fromthe retort and condensed in the cold zone where it later must be chippedofi the walls of the retort. This recovered bulk samarium metal issuitable only for melting stock which is admixed with molten cobalt inproper amount and cast into an ingot. The ingot is then ground to a fineparticle size, ordinarily finer than one micron, to develop itspermanent magnet properties. The ground material may then be compressedin a magnetizing field and sintered to form a solid magnet. A flexiblemagnet may be formed by incorporating the ground material in amagnetizing field in a matrix of an elastomer or polymer.

It is an object of the present invention to produce rare earthintermetallic compounds without the time-consuming, costly steps ofconventional processes. For example, the present invention wouldeliminate the necessity of the separate steps of past processes offorming the rare earth bulk metal, the use of costly lanthanum metal asa reducing agent, and of recovering the rare earth metal itself. Thisobject is shared in US. Pat. application, Ser. No. 849,875, filed Aug.13, 1969, in the name of Robert E. Cech and assigned to the assigneehereof wherein another method of producing novel rare earthintermetallic compounds is disclosed.

Briefly stated, the process of the present invention comprises heating aparticulate mixture of a rare earth metal oxide and calcium hydride toeffect reduction of the oxide. The resulting rare-earth-metal-containingmaterial is fused with cobalt to form the intermetallic compound andthen allowed to solidify. The solid is pulverized and treated to recoverthe cobalt rare earth intermetallic compound.

The oxides of the rare earth metals useful in the present process arethose of the rare earth metals which are the 15 elements of thelanthanide series having atomic numbers 57 to 7l inclusive. The elementyttrium (atomic number 39) is commonly found with and included in thisgroup of metals and, in this disclosure, is considered a rare earthmetal. Mixtures of rare earth metal oxides can also be used.Representative of the oxides useful in the present invention aresamarium oxide sm,0, yttrium oxide v o and misch metal oxides (M mischmetal being the most common alloy of the rare earth metals whichcontains the metals in the approximate ratio in which they occur intheir most common naturally occurring ores.

The rare earth metal oxide can vary in particle size. It is usuallyavailable in commerce in precipitated form, which is the preferred formherein since this form is a very fine particle size, i.e. of the orderof 0.1 micron and highly pure. The smaller the particle size, the fasterthe oxide is reduced.

Since the calcium hydride decomposes in the present process, it may varywidely in particle size and may be as coarse as l2-mesh or coarser.Generally, a pulverized powder is preferred so that an intimate mixtureof the active constituents can be produced. Commercially availablecalcium hydride always contains some calcium oxide. This will notinterfere with proper operation of the process so long as there is asufiicient amount of calcium hydride to reduce the rare earth metaloxide. The necessary excess amount of commercial calcium hydride neededis determinable empirically.

ln carrying out the process of the present invention, the rare earthmetal oxide is admixed with the calcium hydride to affect reduction ofthe oxide. Specifically, using samarium oxide as an example, thesamarium oxide powder is admixed with calcium hydride, and the mixtureis then heated to affect reduction of the rare earth metal constituent.The stoichiometric reaction is as follows:

sm.0.,+ 3c;.1H A 2sm+3cao+3n,.

Although stoichiometric amounts of the rare earth metal oxide andcalcium hydride are satisfactory, it is preferable to use an excessamount of calcium hydride to insure that all of the rare earth metaloxide is reduced to the rare earth metal. The most suitable amount ofexcess calcium hydride can be determined empirically. The product wouldthen contain excess calcium in addition to the rare earth metal andcalcium oxide.

A number of conventional techniques can be used to carry out the instantprocess. Preferably, the calcium hydride and rare earth metal oxide arethoroughly mixed so that in carrying out the reaction, the calciumhydride, which is the reducing agent, can act on the oxide effectively.ln grinding calcium hydride, if any grinding is required, and inhandling the powder mixture, it is essential to use protectiveenclosures so that the atmosphere may be maintained completely free ofmoisture. While calcium hydride is substantially inert in completely dryair, the powder or dust is highly explosive under conditions where anelectrostatic discharge might occur. Therefore, for safetyconsiderations a protective atmosphere such as a nitrogen atmosphere ispreferable to air for mixing and handling the powder. To preventcontamination, the loose powder mixture is preferably placed in a metalfoil bag, e.g., molybdenum or iron metal foil, or a self-supportingmetal pan having a close-fitting cover. Alternatively, the loose powdercan first be pressed into bricks to decrease the volume per unit weightof material, thereby increasing the furnace throughput.

The mixture of calcium hydride and rare earth metal oxide is heated todecompose the calcium hydride and reduce the rare earth metal oxide.Such initial heating should be carried out in an inert atmosphere suchas, for example, argon or helium or a partial vacuum. It can also becarried out in an atmosphere of hydrogen since hydrogen is evolved atthis time. In addition, since hydrogen gas is evolved, this heating canbe carried out at atmospheric pressure. Specifically, at aboutatmospheric pressure when -a temperature of about 850 C. is attained,the reduction process begins as indicated by the evolution of hydrogenand it continues to evolve up to a temperature of about l,0O0 C.Substantially all the rare earth metal oxide is reduced under theseconditions. The product is a fused cake comprised of the rare earthmetal, calcium oxide and usually excess calcium. The rare earthmetal-containing cake, while still hot, may be fused with cobalt, or itmay be cooled in an inert atmosphere prior to fusion. If desired, thisrare earth metal-containing cake can be crushed prior to the fusionprocedure.

The cobalt used in the process can take a number of forms. Preferably,cobalt of melt stock or electrolytic cobalt is used. The fusion ofcobalt or other ferromagnetic metal with the rare earth metal can becarried out by a number of techniques. One technique comprises meltingthe cobalt, then mechanically pushing the rare-earth-containing materialin cake or crushed form under the surface of the molten cobalt to fusethe cobalt and rare earth metal to form the intermetallic compound. Theperiod of time necessary to carry out the fusion can be determinedempirically. The amount of cobalt or other ferromagnetic metal useddepends on the intermetallic compound to formed, therefore, astoichiometric amount of cobalt or other ferromagnetic metal should beused. For example, for the formation of Co R where R is a rare earthmetal the stoichiometric reaction is as follows:

The fusion temperature may vary but it should be such as to maintain thecobalt in a molten form. Fusion should be carried out under an inertatmosphere such as argon or helium. It may be carried out at atmosphericpressure if a minor amount of calcium metal is present. For bestresults, however, it should be carried out at a pressure higher thanatmospheric because excess calcium metal, which is usually present in asignificant amount, has a high vapor pressure. To carry out the fusior apressure induction melt furnance is preferred. Upon completion of thefusion, the rare earth intermetallic compoundcontaining product can beallowed to solidify, preferably in an inert atmosphere to preventoxidation of the rare earth metal. This product can also be cast in aconventi o n al manner to the desired form such as an ingot.

The solid rare earth intermetallic compound-containing product is thenpulverized. This may be done by a number of conventional techniques suchas, for example, crushing the solid with a jaw crusher or a Diamondmortar. The crushed product may then be ground in a conventional mannersuch as by a ball mill or ajet mill.

To recover the cobalt rare earth intermetallic compound particles, avariety of separation techniques can be employed. In one technique amagnetic separator can be used to attract the cobalt intermetalliccompound particles, leaving the calcium oxide. In another method, wateris added to the particulate product to convert the calcium oxide tocalcium hydroxide which is a flocculate precipitate that can beeffectively decanted off with repeated washings with water. A preferredfinal cleanup treatment comprises admixing dilute acetic acid with therecovered cobalt intermetallic compound particles to leach away tracesof remaining calcium hydroxide. The cobalt rare earth intermetalliccompound particles can then be rinsed with water and dried in aconventional manner.

In the present process, if desired, the calcium hydride can be formed insitu by a number of methods. One method comprises admixing calciumcarbide with the rare earth metal oxide and heating the mixture in thepresence of hydrogen to form the calcium hydride. In another methodmagnesium chips or powder are admixed with calcium oxide and heated inhydrogen to form calcium hydride and magnesium oxide which can remain inthe mixture until completion of the process. Once the calcium hydride isformed in situ, the process then can proceed in the same manner as ifcalcium hydride had been added initially.

The present process is useful in forming cobalt rare earth intermetalliccompounds, and particularly Co R compounds, which are useful inpreparing permanent magnets. In the present process, alloys of cobaltwith other ferromagnetic metals may be used instead of cobalt alone toproduce rare earth intermetallic compounds which are particularly usefulin forming magnets. Representative of such alloys are those of cobaltand iron, alloys of cobalt, iron and manganese, and alloys of cobalt andmanganese. In addition, in this invention, iron can be used instead ofcobalt to produce the iron-rare earth intermetallic compound desired.Likewise, alloys of iron with other ferromagnetic metals, can also beused such as, for example, alloys of iron and manganese. Again, theresulting rare earth intermetallic compounds are particularly useful informing magnets.

All parts and percentages used herein are by weight unless otherwisenoted and where screen size is referred to, it is the U.S. Standardscreen size.

The invention is further illustrated by the following examples.

EXAMPLE in this example, in preparing the formulation, a quantityadjustment factor of 0.1445 was used. The formulation was as follows:

Sm O (precipitated, 99.9% pure) 348.86 (mol.wt.Sm,0,-,

0. l445=grams. Calcium Hydride (l4-mesh) =42.l (mol.wt.CaH F3 g.

moles X 1.8 (L8 X stoichiometric req.) 0.l445=32.84 g.

The constituents of the formulation were placed in a plastic bag under anitrogen atmosphere and blended manually until a thoroughly blendedmixture was obtained. The mixture was then placed in a molybdenum foillined iron foil envelope. The envelope was placed in a closed end clearfused silica tube attached to a vacuum system. The system was evacuatedto remove air and refilled with hydrogen gas. The tube, still attachedto the vacuum system, was placed in an air atmosphere tube furnace andthe temperature was raised from room temperature to l,l00 C. in 40minutes. Hydrogen gas began to evolve when a temperature of about 850 C.was reached and continued until the final temperature of l,l00 C. inhydrogen to insure the complete reduction of samarium oxide. The systemwas then evacuated and refilled with helium before withdrawing the tubefrom the furnace and allowing it to cool to room temperature.

The product of this reduction process was a fused cake comprised ofsamarium metal, calcium oxide, and excess calcium metal. Assumingcomplete reduction of samarium oxide to metal the cake would contain43.45 g. of samarium.

The cake was then placed in the feeding tray of an inert atmospherepressure induction melting furnace. 80.7 grams of cobalt-melting stockwere placed in an alumina crucible and placed in the furnace which wasmaintained at 9 atmospheres of argon pressure and a temperature of aboutl,500 C. When the cobalt was completely melted, the cake was droppedinto the melt and pushed under the melt surface to facilitatedissolution of the cake in the melt.

When dissolution of the cake was complete, as evidenced by itsdisappearance from the surface of the melt, the resulting melt productwas allowed to cool in the crucible in the furnace and solidify under aninert atmosphere. The crucible was then broken, and the solid productremoved therefrom. The product was crushed with a hammer to -20-mesh andfurther pulverized by ball milling for 2 hours under mineral spirits.The powder was then stored under the same mineral spirits.

A portion of the stored powder was washed with mineral spirits to removethe calcium oxide that was entrained with the cobalt-samariumintermetallic compound powder. Since calcium oxide was of lower densityand a finer particle size than the cobalt-samarium powder, it remainedsuspended in the mineral spirits after stirring and was decanted off.The mineral spirits-wet cobalt-samarium intermetallic powder wasrecovered and a portion of it was washed with hexane, dried and examinedby X-ray diffraction and found to contain Co Sm phase, Co -,sm phase anda trace of unreacted cobalt. Another portion of the mineral spirits-wetcobalt-samarium powder was pressed into a green body at 120,000 p.s.i.in a magnetic field of 18 kiloersteds. The green body was in the form ofa cylinder about five-sixteenth inch in diameter and about one-half inchin length. The green body was placed in a protective molybdenum foiltube and heated for 30 minutes at a temperature of l,l00 C. in acalcium-gettered helium atmosphere furnace. The body was partiallysintered by this treatment. It was then magnetized in a magnetizingfield of 30 kiloersteds. The resulting magnet was found to have an opencircuit flux density of 2,720 gauss and an intrinsic coercive force H,of 6.5 kiloersteds. From these values a minimum energy product wasdetermined and found to be 3.75Xl0 gauss-oersted.

Another portion of the stored powder was washed with acetone to removethe mineral, spirits then washed with water to convert calcium oxide toa fiocculant precipitate of Ca(OH) The Ca(OH) was then removed byrepeated washes in water and the residual calcium hydroxide dissolved bya final wash with dilute acetic acid. The powder was then washed withwater, alcohol and acetone, and placed under hexane.

The hexane-wet powder was pressed into a green body at l20,00 p.s.i. inan aligning magnetic field of 18 kiloersteds. The resulting cylindricalcompact, weighing about 3 grams and having a diameter five-sixteenthinch and a length of fivesixteenth inch, was placed in a close fittingmolybdenum foil capsule together with 0.1 gram of calcium metal and itwas heated to a temperature of l,lO C. for 5 minutes in acalcium-gettered helium atmosphere. The calcium-infiltrated compact waselectroplated with copper in a cynanide bath to protect it fromoxidation. It was then magnetized in a field of 30 kilogauss. Theresulting magnet was found to have an intrinsic coercive force H, of 7.2kiloersteds, and open circuit flux of 3517 gauss and an estimated energyproduct Bl-i,,,,,, =6 l0 gauss-oersteds.

The estimated energy product was arrived at from measurements of theopen circuit flux of the sample, its intrinsic coercive force, and itsdimensions. On a plot of B vs H, the open circuit flux point was plottedon a load line corresponding to the sample shape, and the intrinsiccoercive force point was drawn on the line B =H in the third quadrant.This point and the open circuit point were connected with a straightline. The rest of the demagnetization curve was approximated by astraight line from the open circuit point to the H=O axis with a slopeof 45. The maximum energy product on this line-segment demagnetizationcurve is the estimated energy product.

What is claimed is:

l. A process for preparing a rare earth intermetallic compoundcomprising heating an intimate particulate mixture of a rare earth metaloxide and calcium hydride to decompose said calcium hydride and therebyefiect reduction of said rare earth metal constituent, contacting theresulting solid rare earth metal-containing material with a metallicmaterial in molten from selected from the group consisting of cobalt.iron, alloys of cobalt and iron, alloys of cobalt, iron and manganese,alloys of iron and manganese, and alloys of cobalt and manganese in aninert to form the rare earth intermetallic compound of said rare earthmetal and said metallic material, allowing the resulting rare earthintermetallic compound-containing product to solidify, pulverizing theresulting solid product and recovering the rare earth intermetalliccompound therefrom.

2. A process according to claim 1 wherein said metal group member iscobalt.

3. A process according to claim 1 wherein the rare earth metal oxide issamarium oxide.

4. A process according to claim 1 wherein calcium hydride is used in anamount in excess of stoichiometric.

5. A process according to claim 1 wherein said calcium hydride is formedin situ in said mixture.

it i t i i

2. A process according to claim 1 wherein said metal group member iscobalt.
 3. A process according to claim 1 wherein the rare earth metaloxide is samarium oxide.
 4. A process according to claim 1 whereincalcium hydride is used in an amount in excess of stoichiometric.
 5. Aprocess according to claim 1 wherein said calcium hydride is formed insitu in said mixture.